An antenna is used to transmit and receive electromagnetic energy typically in the form of radio frequencies upon which have been modulated telephony and data communications.
In this specification most of the disclosure will relate to antennae suitable for use from a mobile platform for facilitating communication between that mobile platform and a satellite. However, it is not the intention of the inventors to imply any limit in the way the antenna according to the invention could be used. It is not inconceivable for the antenna the subject of this disclosure to be useful in radar applications, cellular mobile and base station communication systems and others including fixed and mobile platform applications. Some discussion of these alternative uses will be given but the majority of the disclosure will relate to the mobile-satellite environment. One or more combinations of features described herein are likely to be useful in other applications and the mobile-satellite examples are provided mainly by way of illustration of those features.
Satellites provide an advantageous location for one or more radio communications transponders and a variety of types of land based antennae have been used to receive and transmit to them.
Satellites permanently positioned some 36,000 kilometres above the equator are geographically stationary hence they are referred to as geo-stationary satellites. Geo-stationary satellite antennae are orientated towards the earth and transmit to and receive radio frequency energy to and from a predetermined area of the earth's surface, commonly referred to as its footprint.
A fixed location transceiver within a geo-stationary satellite's particular footprint uses an antenna that is orientated directly towards the relevant satellite. An antenna that receives and transmits signals to and from distant satellites is designed to have maximum gain in a particular direction (unidirectional). Therefore it is necessary to accurately mechanically orientate that antenna so that the pattern of its maximum gain (transmission and reception) is orientated towards a particular geo-stationary satellite. Antennae with large gains are preferable however these will typically have large dimensions and a specialized shape and a narrow maximum gain beam width (power spread of the optimum transmit and receive signal to and from the antenna) which thus requires very accurate alignment with the satellite. Parabolic antennae are an example of such an antenna and are typically used by fixed location users of geo-stationary satellites.
The intervention of adverse weather can reduce the amount of signal received by any type of antenna but it is particularly disadvantageous at frequencies used in satellite systems. Consequently geo-stationary antenna are designed to have the highest possible gains to minimise these effects which thus necessitates large dimensions and narrow beam widths.
Mobile users are also able to use geo-stationary satellites. However, their mobile transceiver equipment must be of the highest quality and more importantly the antenna used with that equipment must be capable of receiving and transmitting at all times in the direction of the satellite which obviously is moving relative to the mobile user.
Some very bulky and expensive parabolic antennae exist for mobile users but typically the mobile user is ideally and typically forced to be stationary while communicating via the satellite with that type of antennae.
There do exist, antenna that can be used from a moving platform (eg a vehicle). One such form of antenna is a very expensive, electrically steerable antenna. This type of antenna however is typically of lower gain that other alternatives since it trades-off signal transmission and reception efficiency for a low profile and convenient operation.
In a mobile environment it is preferable to use an antenna that can quickly re-oriented itself without operator intervention as the mobile user moves throughout the footprint of one satellite antenna or has to change satellites as it moves to another footprint. The antenna will also need to change its orientation when the vehicle moves over uneven ground or changes direction. It may also need to change its orientation quickly back and forth between satellites as it travels in the region of an overlap between two satellite antenna footprints. The latter process of changing satellites is called hand-over and is handled in a number of ways by not only the mobile but also the satellite system, the most common methods of hand-over being referred to as soft and hard.
In geo-stationary satellite communication systems hand-overs occur less often than in telephony cellular systems due to the very large footprints of geo-stationary satellites but a mobile antenna must contend with the other design considerations described above none the less.
Low earth orbit (LEO) satellites circle some 250-1500 kilometers above the earth's surface, and their footprints are smaller than geo-stationary satellites and are also continually moving across the earth's surface swathe like. LEO satellite communications systems provide an alternative to geo-stationary satellite communications systems. However, many more low-earth satellites than geo-stationary satellites are required to provide adequate coverage of the earth's surface. LEO satellites relay radio communications between fixed and mobile users via a system that is also connected to the Plain Old Telephone System (POTS), also known as the Pubic Switched Telephone Network (PSTN). They can also provide features akin the increasingly feature rich cellular digital networks including access to the global computer network commonly referred to as the Internet.
However, the communications system, which supports a LEO satellite relay function is substantially more complicated than that of the geo-stationary satellite relay function as discussed very briefly above. A LEO satellite system uses some 40 to 70 satellites to provide overlapped footprints that simultaneously provide cover over the majority of the surface of the earth (the polar regions are sometimes excluded). Thus, a user wherever they may be, will typically be within the footprint of at least two and ideally three or more low-earth orbit satellites at any one time.
Thus, in a low-earth orbit satellite system both stationary and mobile users it is preferable for their antennae to be able to track the path of the satellite providing the best signal available at any particular time. However, in practice a relatively low gain dipole antenna is used that consequently requires a high transmit power and sensitive receiver.
Both mobile and LEO support systems need to implement a well structured hand-over mechanism so that a link involving for example a telephony conversation can be handed over from one satellite to another with little or no loss of continuity or intelligibility to the users of the system.
Thus the complexity and power requirements of a LEO mobile and satellite transceiver is greater so that it can handle frequent hand-overs.
Preferably therefore, a mobile antenna for a LEO system will have both frequency and directional agility.
Since a low earth satellite is closer to the earth's surface the signals received by users on the earth's surface are much greater than those received from geo-stationary satellite users. Thus mobile antennae used in LEO systems can be smaller and have less gain than those used by geo-stationary satellite mobile users.
Thus regardless of the satellite system being used, desirable features of mobile antennae include Omni-directionality of transmission and reception in the horizontal plane as well as exhibiting appreciable gain in the full range of azimuth angles.
In this specification the term azimuth is typically used to describe the angle of elevation, of a beam of the radiation of an antenna, above the horizontal plane. The horizontal plane is orthogonal to and centred on what is also typically the vertical axis of the antenna with respect to the earth's surface. The horizontal plane is nominally centred at or near the base of the active element of the antenna or is coincident with the electrical ground plane of the antenna. However, the antenna disclosed in this specification may be used, as will be explained, in many configurations. Consequently in use, the longitudinal axis of the antenna will not always be vertically orientated with respect to the earth's surface. In some applications it may be used upside down and in others it may be used on its side, all with respect to the earth's surface. Thus the angle of elevation, referred to herein as its azimuth, is relative to the plane orthogonal to the longitudinal axis of the antenna.
That is to say, a satellite antenna would be ideal if it did not matter what orientation the antenna had it always provides its greatest gain in the direction of an appropriate satellite. This however, is an ideal and is not achievable in practice using existing mobile antenna technology.
An antenna type with some of these features is the helical antenna and the type of helical antenna most commonly used for mobile satellite communications is the quadrafilar helix antenna.
U.S. Pat. No. 5,489,916 to Waterman et al. is an example of such an antenna, which comprises four parallel conductive helices extending about a common vertical central axis. The helices have a common direction of turn about the axis, a common pitch, and a common length between opposite ends. The helices are uniformly radially spaced from each other by 90°, and a single non-conductive (dielectric) helix concentric with the common axis having a pitch much greater than the conductive helices, lies within and supports the conductive helices at a nominal diameter. A casing containing all the helices is secured to one end of the non-conductive helix. A radio frequency tuning device is secured to the other end of the non-conductive helix as well as the conductive helices and is rotatable with respect to the casing. Rotation of the tuning device rotates the non-conductive helices, which alters the common pitch of the conductive helices without substantially varying of the nominal diameter of the conductive helices. The spacing and angle between the common helices remains uniform throughout its length.
The Waterman et al. antenna configuration allows a circularly polarised quadrifilar helix antenna to transmit and receive tuned frequency beams Omni-directionally having maximum gain at a selectable angle in elevation relative to the horizontal (the azimuth angle). For example, a maximum gain beam at an elevation angle of 60° may be required to efficiently receive and transmit to a low-earth orbit satellite at one moment and at the next moment at an elevation angle of 30°. Such agility is required so that the same antenna can receive and transmit with two low-earth orbit satellites until the transceiver can determine which LEO satellite is in a position to provide the strongest signal. In general as the pitch of the conductive helices increases the angle of elevation above the horizontal of the beam of maximum gain increases and it is the controlled changing of the pitch which is used to alter the radiation pattern of the quadrifilar antenna.
Since a mobile is likely to be moving relative to the satellite, be it a geo-stationary or low-earth orbit satellite, it is preferable for the mobile's antenna to be agile in its ability to alter the elevation angle of the beam having the greatest gain. Elevation angle agility is also required to account for times when the mobile antenna moves off the vertical. Such as, for example, when the vehicle to which the antenna is fitted, traverses undulating ground, more so since the nature of this type of reorientation of the antenna is not predictable and can be very dramatic. may receive unwanted signals and/or deplete transmit efficiency by transmitting signals in unwanted azimuth angles.
Further it is of some importance, for the elevation angle of the main beam to be selectable over as great a range as possible. The ideal being between 0° and 90° to ensure that both the sky directly above and the horizon are traversed either during a general scan or during relative movement.
Some of these ideals are not achievable with antennae of the prior art certainly not in the one antenna. It is an aim of this invention to provide an antenna that reduces or eliminates some of the shortcomings of the prior art or at least provides an alternative mechanism for creating them.